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Oct. 3, 2011 -- More than 1 in 10 parents of young children don't follow the recommended vaccination schedule, new research shows.

They decline some vaccines, delay others, or in other ways tweak the recommendations, the survey found.

The 1 in 10 finding is concerning, says researcher Amanda Dempsey, MD, PhD, MPH, assistant professor of pediatrics and communicable diseases at the University of Michigan in Ann Arbor. "That's enough to cause increases in vaccine-preventable disease outbreaks," she says.

Vaccination Schedules & Parents: Survey Findings

Dempsey and her colleagues conducted a survey over the Internet of a nationally representative sample of parents of children ages 6 months to 6 years old. They asked them which vaccination schedule they used and whether their child got all recommended vaccines.

The parents answered other questions, including whether they had a regular health care provider for their child.

Dempsey's team evaluated 748 responses. The parents ranged from 18 to 59 years old, but most were ages 30 to 44.

Some of the 13% who followed an alternative schedule looked to well-known ones, such as those promoted by Dr. William Sears or Dr. Donald Miller. But much more commonly, parents on the alternative schedule tweaked it themselves or took a friend's advice on how to do so. Some said they worked with the child's doctor to develop the alternative schedule.

The patterns among those not following the recommended schedule varied. Among them:

17% said their child did not get any vaccines.

53% said they didn't get some vaccines.

55% said they delay some vaccines until older than the recommended age.

36% said they wait longer between multiple-dose vaccines than is recommended.

The vaccines least likely to be refused were polio and diphtheria-tetanus-acellular pertussis, each refused by 6% of parents.

"Our study is in the ballpark of what was shown before," Dempsey says. "A lot of parents are on the fence about vaccines and this is going to continue to be a problem."

Also troubling, she says, are some other findings. "We found almost a third of parents who are on an alternate vaccine schedule started out on a recommended schedule." And more than a fourth of those still following the recommended schedule said they thought delaying vaccines was safer.

Those most likely not to follow the schedule didn't have a regular health care provider. However, Dempsey isn't sure which situation came first. She doesn't know if those who tweak the schedule have a difficult time finding a doctor or that these parents who don't get regular health care are the ones who tend not follow the schedule.

Dempsey reports compensation for serving on an advisory board for Merck related to the male human papillomavirus vaccination. The company did not participate in this study.

Alternative Vaccination Survey: Perspective

Another expert sees reason for concern about the 13%. "People who refuse vaccines tend to be clustered geographically," says Saad Omer, PhD, MPH, MBBS, assistant professor of global health, epidemiology and pediatrics at the Emory University Schools of Public Health and Medicine and the Emory Vaccine Center.

That, in turn, can create what he calls a ''critical mass" of people to trigger a disease outbreak.

"There is a reason why there is a schedule," says Omer. "The risk of preventable disease is not constant. One of the reasons we give vaccines at a certain age is the children are vulnerable at a certain age."

Another problem, he says, is that as parents spread out the vaccinations, the risk of not completing the recommended ones increases.

The CDC maintains a schedule of recommended vaccines on its web site, www.cdc.gov/vaccines/recs/schedules/child-schedule.htm

Under herd immunity, when a large part of the population has been vaccinated, it is thought to provide some protection for those who have not been vaccinated nor had the disease.

However, Luthy says herd immunity ideally is meant to protect children who can't get vaccinated -- due to being organ transplant recipients, for instance. When parents of children who could be vaccinated decide not to, she says, "we are putting the lives of some of these other children at risk."

Another Perspective

Barbara Loe Fisher, co-founder and president of the National Vaccine Information Center, says the finding of more than 1 in 10 parents changing the schedule is not a surprise to her. Her organization, she says, encourages informed decision making about vaccines.

As the number of childhood vaccines rises, she says, today's parents are becoming more informed and educated. They are doing research on vaccines before deciding whether their child should get them.

Fisher says she is troubled by the conclusions of the researchers that strategies are needed to reduce concerns of parents. Instead, she says, pediatricians ''need to listen to the mothers, most of the time it's mothers, when they describe vaccine reactions."

Some people are more prone to these than others, says says. "Much more needs to be known about vaccine high-risk events."